Phase Optimization Technique in High-Speed Simultaneous Bidirectional Links

ABSTRACT

A bidirectional transceiver includes a transmitter and a receiver that respectively transmits a local signal to and receives remote signal from a common bidirectional communication channel, thus the bidirectional channel signal is the superimposition of the local and remote signals. The bidirectional transceiver also includes a transmit canceller that substantially removes the local transmitted signal from the superimposed signals on the bidirectional channel before the local receiver. The remote signal is transmitted by a remote transceiver over the bidirectional channel. A sampling phase is set, based on timing information in the received remote signal, and the received signal is sampled. Timing relation of transitions in the local transmit signal relative to the receiver sampling phase is set such that transmit signal cancellation is optimum at receiver sampling phase, by changing the delay applied to the local transmit signal. To keep the timing relation of the local transmit signal relative to the remote transceiver, a second delay is applied to the local transmit signal before transmission into the bidirectional channel that provides a delay substantially same as the first delay but opposite in direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. patent applicationSer. No. 15/286,538, filed Oct. 5, 2015 titled “Phase OptimizationTechnique in High-Speed Simultaneous Bidirectional Links”, which claimsbenefit priority to U.S. Provisional Patent Application Ser. No.62/238,109, filed on Oct. 6, 2015 titled “A Receive Sampling PhaseOptimization Technique in Multi-Gbps Bi-Directional Serial Links”, theentire contents of each such application are incorporated by referenceherein.

TECHNICAL FIELD

The present disclosure relates generally to the field of communication,and specifically to high-speed simultaneous bi-directional serial links

BACKGROUND

One technique for simultaneous bi-directional data transmission isseparate communication channels. This can be inefficient, though, asavailable channel capacity in the opposite direction can be unused. Suchinefficiency can be costly, particularly where bandwidth is limited(e.g., wireless communication), the transmission distance is long (e.g.,long-haul fiber optic communications), or the number of availablechannels is limited or costly (e.g., high-throughput integrated circuitpackages and printed circuit boards (PCBs).

Another technique for simultaneous bi-directional communication is toassign different carrier frequencies for signals traveling in oppositedirections. The near-side transmit signal can be modulated by onecarrier frequency, and the received signal from the far-side ismodulated by another carrier frequency. The difference between thecarrier frequencies can be enough so the transmit and receive spectrumsdo not overlap and can be properly filtered out at each receiver. Thismethod can be inefficient because it splits the effective availablechannel bandwidth between the two directions, which can bedisadvantageous in high-throughput applications. Additionally, therequired frequency separation margin, for proper channel isolation andfiltering of signals in each band, can lead to wasteful use of thechannel bandwidth. For example, operation of a simultaneousbi-directional communication using separate frequency channels in eachdirection can waste half or more of the total available channelbandwidth.

Another technique for simultaneous bi-directional transmission operatestransceivers at opposite ends of a channel medium, concurrentlytransmitting signals to one another other, through the channel medium,such that the receiver of each transceiver receives a superposition ofthe signal sent by the opposite end transceiver, and the signaltransmitted by its own transmitter. One technique for removing thesignal transmitted by its own transmitter is to generate, locally, areplica of that transmitted signal and then subtract the replica fromthe superposition, then sample the residual. If the replica is exact,the only signal remaining in the residual is the signal from theopposite end transceiver. However, there can be problems with thistechnique, particularly at higher communication bandwidths. Certain ofthe problems can arise from fast slew rate transitions of thetransceiver's own transmitted signal, as these can introduce noise,especially if proximal in time to a trigger or sampling phase of thereceiver's sampler. However, conventional techniques cannot simply shiftthe sampling phase to move it away from transitions in the locallytransmitted signal, because the sampling phase is optimized in relationto the symbol period in the received signal. Merely shifting thesampling phase could result in sub-optimal sampling, which in turn couldproduce unacceptably high error rates.

There is a need therefore for a practical, stable performancesimultaneous bi-directional transceiver system in which opposite endtransceivers can each cancel receipt of their own transmissions, whilemaintaining in the each of the transceivers an optimal sampling of thesignal received from the other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Illustrates one example bidirectional communication link over asingle physical channel.

FIG. 2A Illustrates a transmitter eye diagram for a non-return to zero(NRZ) data pattern before the physical channel.

FIG. 2B Illustrates an attenuated received eye diagram after a low-passchannel, with the strong local transmit signal transitioning at middleof received eye opening.

FIG. 2C Illustrates an attenuated received eye diagram after a low-passchannel, with local transmit transition timing shifted away from thereceived eye opening.

FIG. 3A Illustrates an example implementation of a phase optimizingsimultaneous bidirectional link according to various aspects.

FIG. 3B Illustrates another example implementation of a phase optimizingsimultaneous bidirectional link according to various aspects.

FIG. 4A Illustrates an example alternative implementation of a phaseoptimizing simultaneous bidirectional link according to various aspects.

FIG. 4B Illustrates another example alternative implementation of aphase optimizing simultaneous bidirectional link according to variousaspects.

FIG. 5 Illustrates an example of another alternative implementation of aphase optimizing simultaneous bidirectional link according to variousaspects.

FIG. 6 Illustrates an example of another alternative implementation of aphase optimizing simultaneous bidirectional link according to variousaspects.

FIG. 7 is a flow diagram of operations to adjust delay elements in animplementation of a phase optimizing simultaneous bidirectional linkaccording to various aspects, to optimize sampling points in accordancewith disclosed concepts and aspects thereof.

DETAILED DESCRIPTION

FIG. 1 illustrates an example simultaneous bi-directional transceiversystem capable of providing recovery, at each of the bi-directionaltransceivers 101 and 102, of the signal transmitted through the physicalchannel 103 by the other. Transceiver 101 includes transmitter 104,receiver 105, transmit replica unit 106, signal adder 107, and a clockand data recovery (CDR) unit 108. Transceiver 102 is similarlyconfigured, and therefore labeling is omitted. Operation will bedescribed for the transceiver 101, and will be understood to apply inlike manner to transceiver 102. In operation, the line signal at thebi-directional transceiver 101 is a superposition of the transmit signaloutput by its own transmitter 104 and a signal received, aftertravelling through the channel 103, from the transmitter of transceiver102. The transceiver 101 removes its own transmit signal from thesuperposition by configuring the transmit replica unit 106 to generatesubstantially the same signal as transmitter 104 but with oppositepolarity, then using signal adder 107 to add the opposite polarityreplica signal and the channel signal and outputs the sum as a residualto the receiver 105. Operations of the CDR unit 108 will be described ingreater detail in reference to FIGS. 2A and 2B. Referring to FIG. 1,assuming the transmit replica unit 106 generates an accurate replica(with opposite polarity), the residual will include none of the signaltransmitted by transmitter 104. However, as the communication signalbandwidth increases, certain phenomena can arise, including significantattenuation by the physical channel 103 of the signal from the oppositeend transceiver, that can complicate, or degrade, or both, theabove-described transmit signal cancellation.

Referring to FIGS. 2A and 2B, information represented by the drawingswill be described referring to FIG. 1, and will assign transceiver 101as a “local” transceiver and transceiver 102 as a “remote” transceiver.Referring to FIG. 2A, illustrated is a transmit signal eye diagram atthe remote transceiver (e.g., at transceiver 102) before the channel103. FIG. 2B shows the attenuated received signal, as received at thelocal transceiver (e.g., transceiver 101) after passing through thechannel 103. At the local transceiver, the CDR unit 108 determinesoptimum sampling phase. A conventional CDR unit extracts the phase andfrequency information from the received signal and generates a localclock that is frequency locked to the received signal with a samplingphase for highest received SNR. In a high-speed simultaneousbidirectional link, the received signal eye opening is significantlysmaller compared to the local transmit amplitude on the same channel.Thus, small residual errors in cancellation of the relatively largetransmit signal leads to large degradations of the received signal.Large transmit cancellation errors specifically occur during the fastsignal transitions, as any minor timing error such as phase error orjitter translates to large amplitude errors. FIG. 2B shows an example ofa highly attenuated received signal with a large local transmit signaltransition at middle of the received eye opening. This conditionrepresents a worst case scenario for this problem, because transmitmaximum slew rate occurs at the same phase location as CDR unit choosesas optimum receive sampling point. Therefore a conventional CDRfunctionality cannot provide an optimum sampling condition for receiveSNR.

Referring to FIG. 1 simultaneous bidirectional link 100, there are twosignals whose phase alignments are important, the conventional CDR andconventional circuit combinations arrangement circuit provide only onedegree of freedom in phase selection. As will be appreciated andunderstood from reading this disclosure, implementations and aspects ofdisclosed methods and systems can provide two degrees of freedom. One isthe control of timing between sampling clock phase and received signal.The second is shifting the transmit transitions of the local transmittedsignal away from the received signal optimum sampling time selected bythe CDR. This condition is shown in FIG. 2C, where the earlier transmittransitions (occurring at middle of the receive eye) are shifted by halfa symbol time to have the maximum distance from the received center eye.In an aspect, the additional timing control can be configured to shiftthe fastest transmit slew rate region (around transmit zero crossings)in between two consecutive optimum receive sampling points. Suchrelative phase adjustment, as shown in FIG. 2C, can align the lowestslew rate portion of the transmit signal with the receiver optimumsampling point, thus minimizes the transmit cancellation error.

FIG. 3A shows an implementation of an example simultaneous bidirectionallink 300 according to various aspects. Referring to FIG. 3A a delay unit302, which will be referred to as a “relative delay” unit 302 isarranged in a first bidirectional transceiver 304 that, when controlledas will be described, sets the relative delay between the local transmittransitions and the optimal receiver sampling phase. By providing thisnew degree of freedom between the local transmit and receive timing, therelative delay unit 302 can shift local transmit transitions away fromoptimal receiver sampling phase as determined by a timing or clockrecovery unit 306. Referring to FIG. 3A, there can be a second or remotebidirectional transceiver 308 on the other side of the bidirectionalchannel 310. In an implementation, as illustrated in FIG. 3A, theconfiguration and components of the remote bidirectional transceiver 308may be identical to or substantially the same as those of the firstbidirectional transceiver.

Accordingly, to avoid obscuring relevant details in FIG. 3A, explicitlabeling and numbering of items in the remote bidirectional transceiver308 is omitted. In some embodiments, each of the items is implicitlynumbered and labeled identical to the corresponding item of the firstbidirectional transceiver 304.

Another application of the bidirectional delay line 312 is to provide ameans for removing the differential skew in the positive and negativepolarities of the received signal. As a differential signal travelsthrough a channel, the positive and negative polarities of the signalexperience different delays as they travel through separate paths. Suchdelay difference is not noticeable at lower data rates, because it is asmall percentage of the each data symbol. However, such delay differencebecomes a larger portion of a data symbol at very high data rates andwill affect signal quality at the receiver severely. This delaydifference can be removed in the same transceiver architectures using abidirectional delay element as shown in FIGS. 3A to 6, for example byapplying different delay adjustment controls to the positive andnegative paths in the bidirectional delay element.

To make this concept easier to follow, FIGS. 3B and 4B are shown indifferential mode. For example, if one assumes the two positive andnegative polarities of the far-end signals are received with a skewdelay between them at transceiver 304, where positive signal is ahead ofthe negative signal by a timing skew of Xt. The bidirectional delayelement 312 can be adjusted such that its positive delay path delays thepositive signal by Xt more than it delay its negative delay path, suchthat at the point the two polarities arrive at the summer 320, the skewbetween the two polarities are substantially cancelled. To avoid thisdelay skew between the positive and negative paths of bidirectionaldelay line 312 affect the transmit signal going out, the positive andnegative transmit signals should be skewed in opposite direction beforebidirectional delay element 312. This opposite differential skew can beapplied by applying different delay skews for positive and negative pathof delay element 302.

In many applications that transmit driver is inherently differential andone cannot apply the differential delay to the output of the driver, adifferential delay element can be added between the transmit driver 316and the summer 320, as shown in FIG. 4B.

The same or similar mechanism as described above in reference to therelative delay unit 302 can exist in a second, or remote bidirectionaltransceiver 308 on the other side of the bidirectional channel 310 inorder to address the same timing adjustment between its own transmitsignal and the received signal from the first bidirectional transceiver304. However, the transmit timing adjustment in the first bidirectionaltransceiver 304 on one side of the bidirectional channel 310 leads to atiming change for the received signal by the remote bidirectionaltransceiver 308 at the other side of the bidirectional channel 310. As aresult, the remote bidirectional transceiver 308 needs to shift itsoptimal receiver sampling phase to track the timing change of the signalreceived from the bidirectional channel, and additionally shift itstransmit timing together with its receiver sampling phase to keep theirrelative timing the same, such that the transmit transitions at receivercontinue to occur in between its receive sampling points. This transmittiming change by the remote bidirectional transceiver 308 leads to thesame timing shift in the signal received by first bidirectionaltransceiver 304, thus negates the original optimum timing adjustmentbetween transmit and received signal, which was set by relative delayunit 302. To address this problem, a timing shift (Delta), opposite inpolarity but substantially equal in magnitude to that of the relativedelay unit 302, can be added to the bidirectional channel 310. This canbe accomplished by arranging a bidirectional delay unit 312 in the firstbidirectional transceiver 304, as shown in FIGS. 3A and 3B. Togetherwith the relative delay unit 302, the bidirectional delay unit 312 canprovide the required relative delay adjustment between local transmitand received signal, while keeping the net delay change of thetransmitted signal into bidirectional channel substantially the same. Inthis scheme, the effective relative delay change between the transmitand receive timing is the sum of the delay magnitude changes (Delta)applied by the relative delay unit 302 and the new bidirectional delayunit.

Since in a normal operation, the delay change (Delta) in the two delayunits (relative delay unit 302 and bidirectional delay unit 312) need tohave substantially the same magnitude, the effective relative delaychange between transmit and received signals at receiver sampling pointis the sum of the delay change of each of the delay units (i.e.,2×Delta). While one bi-directional delay unit is sufficient to addressthe timing conflict between the transceivers on two sides of thebi-directional channel, the remote bidirectional transceiver 308 caninclude a similar bi-directional delay unit 312 to provide wider delayadjustment for the link.

Referring to FIGS. 3A and 3B, the relative delay unit 302 is shownarranged preceding the transmitter 316 and transmit replica unit 318.This arrangement can provide alignment of the output of the transmitter316 to the output of the transmitter replica unit 318 feeding thecanceller unit 310. However, as will be shown in reference to FIGS. 4Aand 4B, the relative delay unit can serve the same function if it isreplaced by one duplicate placed after the transmitter 316 and anotherduplicate after transmit replica unit 318, as long as it does not affectthe receive signal.

FIGS. 4A and 4B show examples where the relative delay is applied aftertransmitter 316 and transmit replica unit 318, by use of a firstrelative delay unit 402 at the output of the transmitter 316, and asecond relative delay unit 404 after the transmit replica unit 318, butbefore the canceller unit 320 in the path to the receiver 322.

Another permutation of such architecture is shown in FIG. 5, where therelative delay is still applied after the transmitter 316 by the firstrelative delay unit 402, but is applied preceding the transmit replicaunit 318 using relative delay unit 502.

Another permutation of the same architecture is shown in FIG. 6, wherethe relative delay is applied preceding the transmitter 316 by the firstrelative delay unit 602, but is applied after the transmit replica unit318 using relative delay unit 604.

Referring again to FIGS. 3A and 3B, for purposes of description, achannel coupling the output of the transmitter 316 to one end of thebidirectional delay unit 302 and to an input of the canceller unit 320can be termed a “local signal channel” (visible, but not separatelynumbered in the drawings). Also for purposes of description, thetransmit replica unit 318 and the canceller unit 320 can be collectivelyreferenced as a “cancellation circuit” (visible, but not separatelynumbered in the drawings). The cancellation circuit, as can beunderstood from description herein of operation of its components, canbe configured to generate a replica of the delayed driver signal, (e.g.,the output of the transmitter 316 in FIGS. 3A and 3B, for example) andto receive a signal from the local signal channel, and furtherconfigured to generate a receiver input signal (to the receiver 322)based on subtracting the replica of the delayed driver signal (from thetransmit replica unit 318) from the signal received from the localsignal channel.

FIG. 7 illustrates an example flow 700 of operations in a method oftiming adjustment between local transmitter transition and receiversampling for best transmit signal cancellation at receive optimalsampling phase in a simultaneous bidirectional link, using system ofFIG. 3A or 3B. Persons of ordinary skill, upon reading this disclosure,can readily apply the flow 700 to equivalent operations using eitherFIG. 4A, FIG. 4B, FIG. 5, or FIG. 6 system. Referring to FIGS. 3A and3B, at step 702 transceiver 304 starts transmission of a local signalinto the bi-directional channel 310, and the cancellation circuitoperates to remove that local transmit signal before being received bythe receiver. At steps 704 and 706, transceiver 304 receives the signalfrom the channel 310 and receiver 322, using CDR unit 306, recovers thebest phase to sample the received signal. Once the best sampling pointwith regards to the received signal is identified, transceiver 304internally determines if the residual local transmit signal has aminimal slope at the receiver sampling phase. If the answer is “yes,” noaction is taken and transceiver continues to track the receive signalphase. Otherwise, at step 710, transceiver 304 uses relative delay unit302 and bidirectional delay unit 312 to shift the timing of the transmittransitions away from the sampling phase at the receiver 322, butwithout shifting the phase of the transmitted signal into channel 310.The relative timing between transmit and receive is selected, at step712, such that the residual local transmit signal has a minimal slope atthe receive sampling phase. As a result, after the adder or cancellerunit 320, receiver sampling occurs at a most stable portion of the localtransmit signal with minimal slope, or effectively at the middle of thetransmit eye. As discussed earlier, at this point the transmit signalcancellation can be performed most accurately and optimally.

It can be noted by persons of ordinary skill, upon reading thisdisclosure that, for a known channel with a constant delay, the optimumrelative delay setting between transmit and receive timing of abidirectional transceiver can be identified a priori and hard-coded inthe relative delay unit and the bidirectional delay unit in advance.Additionally, in certain implementations that the delay of thebi-directional channel itself can be controlled or its delay can beproperly set in advance by design for a target signaling scheme, thebidirectional delay unit can be eliminated from the transceiver.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A bi-directional transceiver circuit comprising:a delay driver circuit, configured to receive a data-in signalcomprising positive and negative polarities with a skew delay betweenthem, and to drive into a local signal channel a delayed driver signal,the delayed driver signal representing the data-in signal by a firsttime shift; a bi-directional delay element configured to couple thelocal signal channel with an external bi-direction channel medium; acancellation circuit, configured to generate a replica of the delayeddriver signal, and to receive a signal from the local signal channel,and further configured to generate a receiver input signal based onsubtracting the replica of the delayed driver signal from the signalreceived from the local signal channel; a sampler, configured to samplethe receiver input signal, at a sampling phase; and a sampling phasecontroller, configured to adjust the sampling phase, based on thereceiver input signal.
 2. The bi-directional transceiver circuit ofclaim 1, wherein the delay driver circuit is a settable delay drivercircuit, further configured to receive a delay control setting, and toset the first time shift in accordance with the delay setting.
 3. Thebi-directional transceiver circuit of claim 2, wherein the delay settingis a delay control signal, and wherein the bi-directional transceivercircuit further comprises: a local transmit phase controller, configuredto generate the delay control signal based on a timing of transitions inthe delayed driver signal relative to the sampling phase.
 4. Thebi-directional transceiver circuit of claim 3, wherein the localtransmit phase controller is further configured to adjust the delaycontrol signal to maintain a given optimal phase difference between thetiming of transitions in the delayed driver signal and the samplingphase.
 5. The bi-directional transceiver circuit of claim 4, wherein agiven optimal phase difference is where the transmit signal is maximallycanceled at the sampling phase.
 6. The bi-directional transceivercircuit of claim 3, wherein: the settable delay driver circuit isconfigured to change the first time shift by an amount Delta, Deltabeing a time shift corresponding to the delay control signal, and thebi-directional delay element, configured to provide a bi-directionalpath, having a second time shift between the external channel medium andthe local signal channel, the Delta change in the second time shifthaving the same or substantially the same magnitude as a magnitude ofthe Delta change in the first time shift having a polarity opposite to apolarity of the first time shift.
 7. The bi-directional transceivercircuit of claim 6, wherein: the settable delay driver circuit includesa driver, having a driver input and a driver output, and includes anadjustable delay element, the adjusting delay element configured toprovide a path having the first time shift, from the data input to thedriver input, and to receive the delay control signal, and in response,to change the first time shift by the Delta, the cancellation circuitincludes an inverter and an adder, the inverter having an input coupledto the input of the driver, and having an inverter output, the adderhaving a first adder input configured to receive signals from the localsignal channel, a second adder input configured to receive an outputsignal from the inverter, and an adder output configured to output fromthe adder output, as the receiver input signal, a sum of the receivedsignals from the local signal channel and the output signal from theinverter.
 8. The bi-directional transceiver circuit of claim 7, wherein:the adjustable delay driver circuit includes a driver and a firstadjustable delay element, the driver having a driver input and a driveroutput, the driver input being coupled to the data-in terminal, thefirst adjustable delay element being configured to provide a first path,having the first time shift, from the driver output to the local signalchannel, and further configured to receive the delay control signal, andin response, to change the first time shift in the first path by theDelta, the cancellation circuit includes an inverter, a secondadjustable delay element, and an adder, the adder having an adder firstinput configured to receive signals from the local signal channel, anadder second input, and an adder output, the inverter having an inverterinput and an inverter output, the second adjustable circuit beingconfigured to provide a second path, having a time shift equal to orsubstantially equal to the first time shift, from the inverter output tothe adder second input, and further configured to receive the delaycontrol signal, and in response, to change the time shift in the secondpath by the Delta or approximately the Delta, and the adder isconfigured to output from the adder output, as the sampler input signal,a sum of the received signals from the local signal channel and theoutput of the second adjustable delay element.
 9. The bi-directionaltransceiver circuit of claim 7, wherein: the adjustable delay drivercircuit includes a driver and a first adjustable delay element, thedriver having a driver input and a driver output, the driver input beingcoupled to the data-in terminal, the first adjustable delay elementbeing configured to provide a first path, having the first time shift,from the driver output to the local signal channel, and furtherconfigured to receive the delay control signal, and in response, tochange the first time shift in the first path by the Delta, thecancellation circuit includes an inverter, a second adjustable delayelement, and an adder, the adder having an adder first input configuredto receive signals from the local signal channel, an adder second input,and an adder output, the inverter having an inverter output coupled tothe adder second input, and an inverter input, the second adjustabledelay element being configured to provide a second path, having a timeshift equal to or substantially equal to the first time shift, from thedata-in terminal to the inverter input, and further configured toreceive the delay control signal, and in response, to change the timeshift in the second path by the Delta or approximately the Delta, andthe adder is configured to output from the adder output, as the samplerinput signal, a sum of the received signals from the local signalchannel and an output from the inverter output.
 10. The bi-directionaltransceiver circuit of claim 7, wherein the bi-directional adjustabledelay element is further configured to: provide the bi-directional path,having the second time shift, between an external wireless channelmedium and local signal channel, receive the delayed driver signal fromthe local signal channel, and transmit into the external wirelesschannel medium a corresponding wireless transmit signal, delayed by thesecond time shift relative to the delayed driver signal, and receive awireless signal from the external wireless channel medium and deliver,into the local signal channel in superposition with the delayed driversignal, a corresponding delayed received signal, delayed by the secondtime shift relative to the received wireless signal.
 11. Thebi-directional transceiver circuit of claim 7, wherein thebi-directional adjustable delay element is further configured to:provide the bi-directional path, having the second time shift, betweenan external wireline channel medium and local signal channel, receivethe delayed driver signal from the local signal channel, and transmitinto the external wireline channel medium a corresponding wirelinetransmit signal, delayed by the second time shift relative to thedelayed driver signal, and receive a wireline signal from the externalwireline channel medium and deliver, into the local signal channel insuperposition with the delayed driver signal, a corresponding delayedreceived signal, delayed by the second time shift relative to thereceived wireless signal.
 12. The bi-directional transceiver circuit ofclaim 7, wherein the bi-directional adjustable delay element is furtherconfigured to: provide the bi-directional path, having the second timeshift, between an external optical channel medium and local signalchannel, receive the delayed driver signal from the local signalchannel, and transmit into the external optical channel medium acorresponding optical transmit signal, delayed by the second time shiftrelative to the delayed driver signal, and receive an optical signalfrom the external optical channel medium and deliver, into the localsignal channel in superposition with the delayed driver signal, acorresponding delayed received signal, delayed by the second time shiftrelative to the received optical signal.
 13. A method comprising:receiving a signal from a remote transceiver, over an external channelmedium; delaying, by a first time shift, the received signal from theexternal channel medium to generate a delayed received signal;recovering timing information from the delayed received signal; settinga sampling phase, based at least in part on the recovered timinginformation; sampling the delayed received signal according to thesampling phase; receiving a data-in signal; delaying, by a second timeshift, the data-in signal to generate a delayed local transmit signal,the delayed local transmit signal including transitions, at a delayrelative to the data-in signal by a time shift amount that places thetransitions at a given phase relative to the sampling phase; delaying,by the first time shift, the delayed local transmit signal to generate atransmit signal; and transmitting into the external channel medium thetransmit signal.
 14. The method of claim 13, further comprising:generating a replica of the delayed local transmit signal; canceling thedelayed local transmits signal, wherein the canceling includessubtracting the replica of the delayed local transmit signal from thedelay received signal to generate a receiver input signal, and samplingthe receiver input signal.
 15. The method of claim 14, whereingenerating the replica of the delayed local transmit signal includesdetecting the phase of the transitions relative to the sampling phaseand adjusting the time shift amount until the detected phase meets anoptimal point.
 16. The method of claim 15, wherein detecting the phaseand adjusting the time shift amount is repeated at a frequency to ensurethe detected phase continues to meet the optimal point.
 17. Thebi-directional transceiver circuit of claim 15, wherein the optimalpoint is where the delayed local transmit signal is maximally canceledat the sampling phase.
 18. The method of claim 13, wherein transmittingthe transmit signal comprises transmitting in response to a change inmagnitude and polarity in the second time shift, changing a magniture ofthe first time shift by an amount equal or substantially equal to thechange in magniture of the second time shift and changing a polarity ofthe first time shift to be opposite to the polarity of the second timeshift.